Genetics Exam 2

alleles

Alternate forms of the same gene

Wild type

-The allele that occurs most commonly in a population
-May be dominant or recessive

mutant allele

-allele caused by a mutation in the wild-type allele
-May specify an alternate gene product or inactivate the gene

null allele

•A gene that does not produce a functional gene product
due to a mutation in the wild-type gene
•Usually recessive

when the mutant allele is dominant

when naming alleles for model organisms, when is upper case used?

complete dominance

The heterozygote is phenotypically indistinguishable from the homozygote

incomplete dominance

Phenotype is intermediate between the two homozygous phenotypes

heterozygote advantage

-gives carriers with one affected allele benefit in another area
-Helps explain why some mutations are maintained in a
population

Pleiotropy

A single gene can present multiple effects

Codominance

Heterozygote shows the phenotypic effects of both alleles equally

polymorphic

name for locus with multiple functional alleles

lethal alleles

•Mutations in essential genes
•May be dominant or recessive

recessive lethal alleles

give 2:1 phenotypic ratio in monohybrid crosses

gene interactions

The situation in which more than one gene is involved in
determining a particular phenotype

epistasis

the effect of one gene pair masks or modifies the effect of
another gene pair

epistatic

•name for the gene that masks
or modifies
•may be dominant or
recessive

hypostatic

The gene whose effects are masked or modified

recessive epistasis

-Two recessive alleles at the epistatic locus prevent expression of the alleles at the hypostatic locus
-results in modified phenotypic ratio

dominant epistasis

•A dominant allele at the epistatic locus prevents expression of the alleles at the hypostatic locus

complementation

The presence of at least one dominant allele at each locus is necessary for wild-type phenotype

Homogametic sex

Produces gametes containing only one kind of sex chromosome

Heterogametic sex

Produces gametes which carry one of two different kinds of sex chromosomes

heteromorphic

chromosomes with different in size and/or centromere position

Thomas Hunt Morgan

•Demonstrated that gene was located on the X chromosome
•First data in support of the chromosome
theory of heredity

sex linkage

Genes located on the sex chromosomes show different patterns of inheritance than autosomal genes

male

in x-linked recessive disorders which sex shows the trait more

a recessive trait is not sex linked

what does a mating involving an affected female in which
some or all sons are not affected indicates

x linked dominant disorders

-Very rare
-Affected males pass on condition to all of their daughters, but none of their sons

a dominant trait is not x linked

what does a mating involving an affected male in which
some or all daughters are not affected indicate?

sex-limited heredity

•Phenotype is autosomally transmitted
•Expressed in only one sex

Sex-influenced heredity

•Autosomally transmitted
•Displayed in both males and females
•Expression is dependent upon hormone constitution, so expression patters differ between males and females

variable expressivity

Individuals with the same genotype differ with respect to the severity of the phenotype

penetrance

The probability that an individual who has the genotype associated with the trait will display the phenotype, affected interactions with the internal cellular environment and the external environment

methylation

how can genes be turned off in genomic imprinting

discontinuous traits

•characteristics with a small number of discrete phenotypes
•Usually determined by one or a few gene loci

multifactorial

Having both genetic and environmental components

Bateson and Yule

who proposed the multiple factor/multiple gene hypothesis

Hermann Nilsson-Ehle

-performed experiments on grain color in wheat that were the basis for multiple factor hypothesis
-proposed additive and nonadditive alleles

additive allele

Contributes a certain amount to the phenotype

nonadditive alleles

Does not contribute to the phenotype

Multiple-factor hypothesis

-Traits are polygenic
-Many genes, each behaving in a Mendelian fashion,
contribute towards the phenotype
-Each gene locus may be occupied by either an additive or nonadditive allele
-Together, the additive alleles produce substantial
phenotypic variation

equal

In the multiple factor hypothesis, the contribution of each additive allele to the final phenotype is approximately _____

continuous traits

Phenotypic variation can lie at any point within a set
range of measurements

meristic traits

Traits in which the phenotype can be expressed as a
whole number

Threshold Traits

-Typically have a small number of distinct
phenotypic classes
-The combination of alleles at multiple loci gives an individual a greater or lesser likelihood of showing one of these phenotypes

complementation analysis

used to determine whether two independently isolated mutations are due to mutations in the same or different genes

the mutations are in different genes

In complementation analysis 2 isolated mutants are crossed, what does wild type progeny indicate?

Bateson and Punnett

Who published the first study of linked genes about sweet pea plants inn 1906?

coupling between dominant and recessive alleles

What did bateson and punnett propose?

Thomas Hunt Morgan

Who hypothesized that parental (non-recombinant) types are more common than non-parental?

They are physically linked

What did Hunt Morgan propose about genes inherited together?

FA Janssens

Who first described chiasmata as the sites of crossing over in 1909?

50%

What is the theoretical limitation to recombination with crossing over?

no

does crossing over always occur between genes?

recombinant frequency

An estimate of the proportion of chromosomes in which a crossover occurred between genes that is proportional to the distance between two linked genes

interchromosomal recombination

-recombination due to independent
assortment
-R.F. = 50%

Intrachromosomal recombination

-recombination due to crossing over
- R.F. < 50%

linkage

The situation where gene pairs reside
on the same chromosome

Linkage group

A group of genes having their loci on
the same chromosome

the number of chromosomes

Number of linkage groups should be
the same as what?

linkage phase

Combination of alleles linked on the
same chromosome

coupling conformation

AB or ab

repulsion conformation

Ab or aB

complete

if only paternal chromosome types are produced linkage is _____

Alfred Sturtevant

Used test cross data to map the relative distance between linked genes

increases

The frequency of crossing over ______ as the distance between two genes increases

three point testcross

Aids in detection of double crossovers (DCOs) and allows relative order of genes on a chromosomes to be established with one cross

AaBbCc x aabbcc

what is crossed in an autosomal 3 point test cross?

AaBbCc female and abc/Y male

what is crossed in a sex-linked 3 point test cross?

NCOs

What progeny type are most common?

DCOs

What progeny type are least common?

in the middle

Where is the gene that is out of phase in the DCOs and NCOs located?

Add the 2 shorter distances together or add 2x the dcos to the formula.

How do you find the RF for the two most distant genes?

Microsatellites (STRs)

• Short sequences (1 to ~6 bp long)
• Tandemly repeated
• Found throughout the genome
• E.g., CACACACA

in length

how do heterozygote microsatellite alelles differ?

Expansion repeat disorder

Disease occuring when the number of copies of a
normally occurring STR increases to a critical
number

minisatellites

Similar to microsatellites
• Tandemly repeated sequence
• Number of repeats varies, constituting different alleles
• Repeat sequence is longer (10 to 100 nucleotides)

Single Nucleotide Polymorphisms

Variations at a single base pair

80

SNPs comprise over ____% of the 0.1% variation in the human
genome

XX/XY

What kind of sex determination do humans use?

protenator mode

XX/X0 mode of chromosomal sex determination

lygaeus mode

XX/XY mode of chromosomal sex determination

heterochromatic

A large portion of the Y chromosome is _______

Pseudoautosomal regions

-On both ends of the Y chromosome
-Share homology with regions on both ends of the X chromosome

synapsis

PARs allow for this to occur during prophase I of meiosis

MSY

Contains a number of genes specific for male development

SRY

-Sex-determining region Y
-Encodes “testis determining factor”

fifth

Up to the _____ week of gestation, male and female embryos are identical

6 to 8

At ____ weeks, SRY becomes active

testes, female

If SRY is not transcribed
• ____ do not develop
• Embryo develops as a _____

Androgen Insensitivity Syndrome

• Reduce response of cells to testosterone and dihydrotesterone
• Male sexual development is suppressed; individuals show phenotypes ranging from normal female external genitalia (CAIS) to normal male external genitalia with infertility

twice

Mammalian females have ______ as many X-linked genes as males in the X-specific region of the X chromosome

Barr Body

- dosage compensation in mammals
- all but 1 x chromosome is inactivated

Lyon Hypothesis

-Inactivation of X chromosome occurs early in development
- Once inactivation occurs, all progeny cells have the same X chromosome inactivated
- Leads to patches of tissues with different phenotypes

Anhidrotic ectodermal dysplasia

• Absence or hypoblastic development of several ectodermal organs including teeth, hair, nails and a variety of exocrine glands including sweat,
salivary, and mammary glands
• Heterozygous females (XAXa) are
mosaics for these traits

genotypic

What mode of sex determination do drosophilia use?

Calvin Bridges

• Studied the progeny of triploid female Drosophila
• Concluded that the ratio of the number of X chromosomes to the number of haploid sets of autosomes determines sex

1X : 1 set of Autosomes

chromosome ratio that produces female fruit flies

ratios higher than 1:1

how are metafemales produced?

1X : 2 sets of Autosomes

chromosome ratio that produces male fruit flies